1. Field of the Invention
The present invention relates to a hole forming method and a laser processing apparatus for forming a laser processed hole in a workpiece configured by bonding a first member formed of a first material and a second member formed of a second material, the laser processed hole extending from the first member to the second member.
2. Description of the Related Art
In a semiconductor device fabrication process, a plurality of crossing division lines called streets are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby partition a plurality of regions where devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the streets to thereby divide the regions where the devices are formed from each other, thus obtaining individual semiconductor chips. For the purposes of achieving smaller sizes and higher functionality of equipment, a module structure having the following configuration is in practical use. This module structure is such that a plurality of devices are stacked and bonding pads provided on each device are connected to each other. In this module structure, through holes (via holes) are formed in a semiconductor wafer at positions corresponding to the bonding pads, and a conductive material such as aluminum is embedded in each via hole so as to be connected to the corresponding bonding pad (see Japanese Patent Laid-open No. 2003-163323, for example).
Each via hole in the semiconductor wafer mentioned above is formed by using a drill. However, the diameter of each via hole in the semiconductor wafer is as small as 90 to 300 μm, so that the formation of each via hole by using a drill causes a reduction in productivity. To solve this problem, there has been proposed a hole forming method for a wafer composed of a substrate and a plurality of devices formed on the front side of the substrate, a plurality of bonding pads being formed on each device, wherein a pulsed laser beam is applied to the substrate from the back side thereof to thereby efficiently form a plurality of via holes respectively reaching the plural bonding pads (see Japanese Patent Laid-open No. 2007-67082, for example).
The wavelength of the pulsed laser beam is selected so as to have low absorptivity to the metal forming the bonding pads and have high absorptivity to the material forming the substrate, such as silicon and lithium tantalate. However, in applying the pulsed laser beam to the substrate from the back side thereof to thereby form the via holes respectively reaching the bonding pads, it is difficult to stop the application of the pulsed laser beam at the time each via hole formed in the substrate has reached the corresponding bonding pad, causing a problem that the bonding pads may be melted to be perforated by the pulsed laser beam. To solve this problem in the hole forming method for the wafer disclosed in Japanese Patent Laid-open No. 2007-67082, there has been proposed a laser processing apparatus such that a laser beam is applied to a material to generate a plasma from the material, and a spectrum caused by this plasma and inherent in the material is detected to thereby determine whether or not the laser beam has reached each bonding pad formed of metal (see Japanese Patent Laid-open No. 2009-125756, for example).